The deepest region of our planet is, obviously, really hard to get to. The Mariana Trench reaches a maximum depth of nearly 11 kilometres (6.8 miles) that we know of. So there's potentially a lot down there that we haven't discovered or seen yet.
This little guy, identified by NOAA scientists as a jellyfish belonging to the Crossota genus, was filmed by high-definition cameras on the remote-operated underwater vehicle Deep Discoverer, equipped to the ship Okeanos Explorer, at a depth of around 3,700 metres (2.3 miles). It's a type of hydromedusae, which typically range from 0.5 to 6 centimetres (0.2 to 2.36 inches) in diameter, but unlike others of its kind, it doesn't have an immobile polyp stage, instead spending its entire life drifting the ocean.
According to Scientific American, the pretty glowy yellow bits? They're probably its gonads. And, as the NOAA notes, the posture of its tentacles at the beginning of the video, evenly extended outwards, suggests that the jellyfish ambushes its prey, motionlessly drifting until something bumps into its grasp.